In various circumstances, a woodwind instrument player (particularly, a saxophone player) may move between a sitting position and a standing position while continuing to play. Obviously, the reverse is equally true, and an individual that has been standing (to play a solo, for example), returns to a sitting position while continuing to play.
When the musician is seated, the may be positioned to his right side, at an angle (for example, many types of saxophones, bass clarinet, or the like). When standing, the musician holds the instrument directly in front of him (i.e., parallel to his body). Obviously, this change in position between the instrument and the musician means that the orientation of the mouthpiece with respect to the musician changes (since the mouthpiece is fixed in place along the neck of the instrument). Thus, if one observes a saxophone player, you may see him take the time to adjust the position of the mouthpiece (sometimes while continuing to play—one hand used to twist the mouthpiece around on the neck, while playing some notes with the other hand).
Clearly, the need for the musician to adjust the position of the mouthpiece while continuing to play is awkward. Additionally, it is possible that the musician may slightly change the position of the mouthpiece a long the neck (unintentionally), which may affect the tuning of the instrument.